Simultaneous line shift and source parameter inversion applied to a scalar magnetic survey for small unexploded ordnance

Abstract

Continuing advances within unmanned aerial vehicles and scalar magnetic sensor technology are revolutionizing geophysical remote sensing. For perspective, examples of high resolution, low noise data presented in this study enabled detection and accurate positioning of human‐made ferrous objects with a total mass of 0.5 kg, from altitudes around and above 2 m. However, with these new possibilities come a requirement of higher fidelity, where good sensor position estimates, in particular, are key to achieving a data quality sufficient for reliable data interpretation and geophysical inversion. Apart from inherent instrument properties, typical factors that hamper sensor position estimation are lag and parallax errors. Lag errors arise from time‐stamping delays between the positioning system and sensor reading, while parallax errors stem from the imperfect recovery of sensor positions due to a decoupling and/or distancing of the positioning system and sensor head. The impact from each of these error sources is typically similar and partially or even completely inseparable. A number of correctional techniques exist, typically based on large‐scale data correlations, or collection of specific additional data. We present a survey data adjustment method based on the simultaneous inversion of source parameters and a number of independent survey shifts or lags in pre‐specified sections of a survey. The method is particularly suited for surveys where the sensor path through space can be assumed well known, but the sensor position itself may suffer lags or shifts relative to the position measurement. The approach is based on modelling an existing source signature within the data and requires no data transformations, such as continuation to a two‐dimensional plane, or enforcing of smoothness. The approach is an amendment to independent source inversion and is ideal for scenarios where the source behaviour is thought to be well known, such as processing efforts in which the objective is source–parameter inversion. In this study, we validate the approach using a simple point‐dipole model to adjust synthetic data and subsequently demonstrate it on an unmanned airborne scalar magnetic survey for small unexploded ordnance targets, where a hardware malfunction resulted in unknown time‐shifts between magnetic sensor readings and positioning unit output. The adjustment procedure was carried out on line data, based on a single anomaly from a 155 mm shell, and enabled the recovery of signatures from several other ferrous unexploded ordnance targets as small as 0.5 kg. Subsequent inversion on the corrected signatures provided unexploded ordnance positions with an accuracy suitable for recovery purposes.

High-Speed Magnetic Surveying for Unexploded Ordnance Using UAV Systems

“…In order to evaluate applications of the approach for UXO detection, target signatures were modelled as point-dipoles, which are known to provide close, robust fits to the scalar magnetic signatures of many UXO, given the data altitudes considered [12,14,15,32,33]. This allowed for a simpler analysis of how well-resolved the source position and moment estimates are using different sensor setups.…”

“…Inversion for model parameters was performed using the probabilistic inversion framework described in [34,35], and validated against the the deterministic inversion approach documented in [12]. All inversions were carried out on the along track difference (ATD) of the sensor data, thereby circumventing the majority of the sensor heading error while enhancing smaller wavelengths [12,14].…”

“…Recent developments within UAV technology have spurred the use of UAVs for magnetic surveying [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. One interesting application of UAV based magnetic surveying is UXO pre-investigations, as the approach has can provide dense coverage with highquality raw data at low ground clearance [12,14].…”

“…Recent developments within UAV technology have spurred the use of UAVs for magnetic surveying [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. One interesting application of UAV based magnetic surveying is UXO pre-investigations, as the approach has can provide dense coverage with highquality raw data at low ground clearance [12,14]. The UAV approach also minimizes UXO detonation risks to on-site operators and risks to equipment and personnel associated with low altitude flights in conventional aircraft [2,12].…”

“…An alternative approach, where sensors are towed beneath the UAV (e.g., via tow cable), can produce low noise magnetic data, but may introduce swing-induced bias [19]. Such bias can be alleviated in part through dedicated sensor positioning (which provides benefits for high-fidelity processing and UXO detection efforts [12,14]), and by towing the sensors at sufficient distance to the UAV platform. Stipulation of a general tow cable length is tricky, as the region which is electromagnetically disturbed by the UAV depends on the UAV platform itself.…”